Chlor-Alkali | Membrane

NMT® Electrodes manufactures Membrane Cell Anodes for use in numerous types of membrane cell electrolysers worldwide. The term “electrolyser” refers to a cluster of individual cells and the arrangement is dependent on the manufacturer of the cells themselves.


Membrane cell electrolysers provide the world’s chlor-alkali and chemical manufacturers with membrane-grade chlorine gas (Cl2), sodium hydroxide (caustic soda) or potassium hydroxide (caustic potash) – depending on the brine composition used, and hydrogen gas. Cells that are operated under slightly positive pressure, allow the chlorine gas generated to be used (in conjunction with sodium hydroxide) to produce other products such as sodium hypochlorite (bleach), hydrochloric acid (HCl) and sodium chlorate.


The primary advantage of membrane cells over other cell types is how economical they are in terms of energy savings. They are also easy to build, easy to operate and easy to maintain.


NMT® Electrodes’ Membrane Cell Anodes are Titanium Grade ASTM B265 coated with a Mixed Metal Oxide coating (IrO2/Ta2O5) which are suitable for use in both Monopolar (when cells of an electrolyser are arranged electrically in parallel) and Bipolar (when the cells of an electrolyser are arranged electrically in series) cell designs. NMT® Electrodes’ Membrane Cell Anodes also exhibit durability, low and stable chlorine overvoltages and low levels of oxygen in the chlorine product.



Background on Membrane Cell


In a membrane cell, an ion-exchange membrane separates the anode and cathode compartments. The separator is generally a bi-layer membrane made of perfluorocarboxylic and perfluorosulfonic acid-based films that is slotted in between the anode and the cathode. The brine is fed to the anode compartment where chlorine is liberated at the anode, and the sodium ion migrates to the cathode compartment through the ion-exchange membrane. Unlike in diaphragm cells, only the sodium ions and some water migrate through the membrane. The unreacted sodium chloride and other inert ions remain in the anolyte (the portion of the electrolyte that is in the immediate vicinity of the anode that is changed in composition by the reactions that occur at the anode). Sodium hydroxide is fed to the cathode compartment, where sodium ions react with hydroxyl ions produced during the course of the hydrogen gas evolution from the water molecules. This forms caustic, which increases the concentration of caustic solution. The hydrogen gas, saturated with water, exits from the catholyte compartment. Only part of the caustic soda product is withdrawn from the cathode compartment. The remaining caustic is diluted and returned to the cathode compartment.


Both basic cell technologies, membrane cells and diaphragm cells, generate chlorine at the anode, and hydrogen together with sodium hydroxide in the cathode compartment. The difference that distinguishes between the technologies is the manner in which the anolyte and the catholyte are prevented from mixing. In a diaphragm cell separation is achieved by a separator, and in a membrane cell by an ion-exchange membrane.



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